This application relates to a method and control that serve to reduce the incidence of flooded starts in a heat pump, and particularly while switching between conventional heating and defrost modes of operation.
Refrigerant systems are utilized to control the temperature and humidity of air in various indoor environments to be conditioned. In a typical refrigerant system operating in a cooling mode, a refrigerant is compressed in a compressor and delivered to a condenser (or an outdoor heat exchanger in this case). In the condenser, heat is exchanged between outside ambient air and the refrigerant. From the condenser, the refrigerant passes to an expansion device, at which the refrigerant is expanded to a lower pressure and temperature, and then to an evaporator (or an indoor heat exchanger). In the evaporator, heat is exchanged between the refrigerant and the indoor air, to condition the indoor air. When the refrigerant system is operating, the evaporator cools the air that is being supplied to the indoor environment. In addition, as the temperature of the indoor air is lowered, moisture usually is also taken out of the air. In this manner, the humidity level of the indoor air can also be controlled.
The above description is of a refrigerant system being utilized in a cooling mode of operation. In the heating mode, the refrigerant flow through the system is essentially reversed. The indoor heat exchanger becomes the condenser and releases heat into the environment to be conditioned (heated in this case) and the outdoor heat exchanger serves the purpose of the evaporator where heat is transferred from a relatively cold outdoor air to the refrigerant. Heat pumps are known as the systems that can reverse the refrigerant flow through the refrigerant cycle, in order to operate in both heating and cooling modes. This is usually achieved by incorporating a four-way reversing valve (or an equivalent device) into the system schematic downstream of the compressor discharge port. The four-way reversing valve selectively directs the refrigerant flow through indoor or outdoor heat exchanger when the system is in the heating or cooling mode of operation respectively. If the expansion device cannot handle the reversed flow, then, for example, a pair of expansion devices, each along with a check valve, may be employed instead.
One control feature that is typically incorporated into heat pumps, is a defrost cycle. Typically, the heat exchanger that is cooling the refrigerant will be subject to icing under certain conditions. A defrost cycle is intended to melt the ice on the evaporator and restore efficient and reliable system operation. In the case of a heat pump operating in a cooling mode, it will be the indoor heat exchanger that could potentially ice, and in a heat pump operating in a heating mode, it will be the outdoor heat exchanger that ices, particularly at lower ambient temperatures. When it is desired to initiate a defrost cycle, the four-way reversing valve that routes the refrigerant through the heat pump in a proper direction for cooling/heating mode would be reversed. Thus, hot refrigerant is sent directly to the heat exchanger that has been subject to icing conditions. Essentially, for the defrost operation in a heating mode, the compressor would drive the refrigerant in a cooling mode direction, and for the defrost in a cooling mode, the compressor would drive the refrigerant in a heating mode direction. In practice, the defrost cycle in heat pumps is most frequently utilized in the heating mode of operation.
Defrost cycles raise reliability concerns in heat pumps due to damage to various system components, such as internal compressor components, as well as system components located on the discharge line such as the four-way reversing valve, check valves, etc. Such damage is predominantly caused by flooded starts. A flooded start can occur due to alternating between a conventional heating/cooling and defrost modes of operation in heat pumps, since when the four-way reversing valve is switched, the duties of the indoor and outdoor heat exchangers are also switched.
As an example, when switching from a heating mode to a defrost mode, the indoor heat exchanger becomes the evaporator. Prior to the defrost cycle, it was a condenser. The outdoor heat exchanger now becomes a condenser, and it was the evaporator before the defrost mode of operation was activated.
The outdoor heat exchanger is now exposed to the hot discharge gas, and the defrost will occur. However, flooded conditions at the compressor suction can also be associated with this defrost operation initiation. The flooded start problem occurs because most of the refrigerant would be located in the indoor coil from the past operation in the heating mode when the defrost cycle is first started. When the four-way reversing valve switches to a defrost mode, and the compressor starts, the liquid refrigerant stored in the indoor coil now moves directly into the compressor suction port. This can cause severe flooded start problems, and as described above, can lead to permanent component damage.
The possibility of having a flooded start would occur again when the system is switched back from a defrost mode of operation to a heating mode.
Further, flooded starts are observed in the cooling mode of operation as well and have similar impact on system reliability.